I would go with Hydrogen. It is only dangerous when mixed with air. It must be remembered that the these famous airship disasters of the past were in massive craft that would suffer different effects than a RC blimp/airship. The huge surface area built up static that had nowhere to go, leaking hydrogen also caused electrical current due to process called Osmosis (probably spelt wrong). lightnening was almost certainly a cause of many disasters but as there are four types of lightening (Sky to ground, ground to sky, sky to sky and outer atmosphere/space to sky) it may not have been apparent at the time that an airship was struck by lightening when there were no thunderstorms around (I believe this to be the fate of HMA - NS11 and the African Airship). I do not however think that a RC Airship is likely to be that heavily charged as to attract a bolt of lightening, nor a small blimp.
Something else that was used in the early days was Coal Gas. Now I have no idea what this is allthough the name does suggest how it is made. This was used in the very early days of Balloons-Dirigibles.